Fresnel lenses, also known as zone plates or Fresnel-zone plates, have long been used to focus electromagnetic radiation. Typically, such a lens comprises many concentric annular rings or zones which provide the desired focusing by selective absorption or selective phase shifting of the electromagnetic radiation. In the absorption mode, selected zones absorb the impinging radiation in a manner which precludes destructive interference at the focal point. Similarly, in lenses designed to operate in the phase shift mode, an appropriate phase shift is imparted to the electromagnetic radiation propagating through selected zones relative to the electromagnetic radiation propagation through other zones so that all of the radiation emanating from the lens is in phase at the focal point.
A variety of materials have been used to fabricate Fresnel lenses which focus electromagnetic radiation selected from the visible and nonvisible portions of the spectrum. Prior art techniques for fabricating Fresnel lenses comprise etching of zones in the planar surface of a material which is transparent to the electromagnetic radiation. This is then followed by depositing absorbing or phase shifting material into the etched zones. While this technique works satisfactorily for some lens applications, it is difficult to provide thin width outer zones in large diameter lenses or in thick lenses. This limitation is significant as it is well known that the spatial resolution of a Fresnel lens may be increased by using short wavelength electromagnetic radiation and by decreasing the width of the outermost zone. Accordingly, it would be desirable to focus X rays through a Fresnel lens in applications where high resolution is desired. The problem with fabricating Fresnel lenses that focus X rays, however, is that the required lens thickness and a thin width outermost zone for high resolution are extremely difficult to achieve.